Transcranial Magnetic Stimulation (TMS) is typically delivered using an electromagnet positioned at the side of the head, the top of the head, or somewhere in between the side and the top of the head. Generally speaking, a single or double standard TMS coil placed on a patient's scalp and operated at a power level at, or slightly above, a patient's motor threshold will directly active neurons from the cortical crowns to the bottom of the cortical gyri—a depth of about 1-3 cm. Using this approach, deeper structures (herein referred to as “subcortical”, even when these deeper areas are histologically layered in nature) are activated only secondarily through intracerebral neural connections. Conventional TMS approaches typically do not reach greater depths. For example, the cingulate gyms, the insula and other subcortical structures are generally not directly accessible for modulation traditional TMS without causing overstimulation of non-target (e.g., more superficially located regions) which may lead to pain and discomfort for the patient). Deep brain modulation cannot be accomplished by simply turning up the power of the stimulating electromagnet, because the intervening tissue, including superficial cortex, will be over-stimulated, causing undesired side effects such as seizures.
Positive outcomes for treatment of depression refractory to drug treatment have been demonstrated with repetitive Transcranial Magnetic Stimulation (rTMS, Avery et al., 2005). rTMS works indirectly, because the superficial stimulation of the dorsolateral pre-frontal cortex is carried by nerve fibers to the deeper cingulate gyms. More effective therapy of depression and treatment of a number of other conditions such as chronic pain, addiction, obesity, and obsessive compulsive disorders may be possible with focused brain stimulation that is capable of reaching depths below the cortex. Devices for providing deep brain stimulation with Transcranial Magnetic Stimulation are described in Schneider and Mishelevich, U.S. Pat. No. 7,520,848 and Mishelevich and Schneider, U.S. patent application Ser. No. 11/429,504. Whether superficial or deep stimulation is being employed, focusing the applied magnetic field during TMS has the potential to improve clinical results. In particular, the ability to stimulate at depth could be facilitated by shaping the profile of the magnetic field of one or more primary stimulating electromagnets, thereby focusing their magnetic fields and more preferentially stimulating a given targeted neural structure.
The magnetic fields used for Transcranial Magnetic Stimulation typically determine both the depth and the size of the region of stimulation. Thus, a more focused magnetic field may be capable of stimulating an area that is also more tightly focused, and may be better controlled by the TMS system.
Transcranial magnetic stimulation of deeper brain regions would benefit from improved focusing of the magnetic field of the primary stimulating electromagnets. Described herein are systems, methods and devices for improving the focus of the primary electromagnets used for Transcranial Magnetic Stimulation, and may allow enhanced stimulation of targeted neural structures.